The invention concerns a drive slip control device for a road vehicle having a twin-circuit hydraulic brake installation with front axle/rear axle brake circuit subdivisions wherein one of the brake circuits' subdivision is associated with the non-driven vehicle wheels and the other is associated with the driven vehicle wheels, and both are designed as static brake circuits whose brake pressure supply is achieved by providing a brake device having two outlet pressure spaces, one associated with each brake circuit. The vehicle is equipped with an anti-lock system operating on the pump-back principle and includes brake pressure control valves associated individually with each of the driven and non-driven vehicle wheels, and two return pumps associated one each with the two brake circuits' subdivisions for returning brake fluid is during pressure reduction phases of the anti-lock control, out of the wheel brake(s) currently subjected to the control system into the brake device output pressure space associated with the particular brake circuit. The return pump of the brake circuit of the driven vehicle wheels is used as the pressure source in the brake pressure build-up phases of the driven slip control and a drive slip control valve is provided for control process connection of the return pump to the brake circuit of the driven vehicle wheels. The drive slip control valve is driven by a drive slip control signal from an electronic control unit controlling the control phases of the drive slip control from a basic position associated with the normal brake operation and anti-lock control operation, in which position the output pressure space of the brake device associated with the brake circuit of the driven vehicle wheels is connected to the main brake pipe of the brake circuit, and into an excited functional position in which this output pressure space is shut off from the main brake pipe while the main brake pipe continues to be connected to the pressure output of the associated return pump. Pressure can be connected to the brake circuit of the driven vehicle wheels by means of the return pump as controlled by a brake pressure build-up control signal from the electronic control unit and a pressure limiting valve is provided to limit the pressure connected to the brake circuit of the driven vehicle wheels during drive slip control operation, by means of which brake fluid can flow out of the brake circuit of the driven vehicle wheels to the brake fluid reservoir of the brake installation when the output pressure of the return pump exceeds a specified threshold value.
A drive slip control device combined with an anti-lock system is known from DE-OS 31 37 287 C2, and comprises a rather complicated structure. An interesting possibility considered for simplifying this known drive slip control device wherein the return pump of the anti-lock system was already used as the reservoir charging pump for the pressure reservoir provided as the auxiliary pressure source within the drive slip control device was to avoid the use of this pressure reservoir and, in its place, to use the return pump associated with the brake circuit of the driven vehicle wheels directly as the auxiliary pressure source for the drive slip control device.
The test drive slip control device designed in accordance with this concept was laid out for a vehicle with a rear axle drive and the anti-lock system of the vehicle corresponding to the series production type, i.e., the design of the return pumps provided for the two brake circuits involved piston pumps which were not self-priming. In order to ensure the functioning of the return pump associated with the rear axle brake circuit as a pressure source in drive slip control operation, the vehicle was equipped with a booster pump by means of which brake fluid was pumped out of the brake fluid reservoir of the brake installation into the pump chamber of the return pump associated with the rear axle brake circuit in drive-slip control operation. The control behavior achieved in this test-type drive slip control operation, however, was unsatisfactory in that the control was very sluggish, i.e., the build-up of brake pressure occurred relatively slowly after the booster Pump and the return pump had been switched on. In order to achieve a response behavior which was at least approximately comparable with conventional drive slip control, it therefore appeared unavoidable to provide a return pump with a larger pumping capacity, at least for the brake circuit of the driven vehicle wheels. There are, however, design limits to such an increase in size so that, for this reason alone, it did not always appear possible to achieve a sufficiently rapid response behavior of the control system, particularly since the return pump of the conventional anti-lock system did not appear to be a suitable auxiliary pressure source for drive slip control.
On this basis therefore, the object of the invention is to produce a drive slip control device of the type mentioned at the beginning, which responds sufficiently rapidly even if a return pump of conventional type is used as the auxiliary pressure source.
The invention achieves this object by having the electronic control unit generate an output signal for driving the drive slip control valve into its excited shut-off position and an output signal causing the activation of the return pump of the brake circuit of the driven vehicle wheels and output signals by means of which the brake pressure control valves and of the wheel brakes and of the driven vehicle wheels are driven into their shut-off position as soon as the drive slip of at least one of the driven vehicle wheels reaches or exceeds a specified threshold value whose magnitude is between the required slip value and the response threshold value of the drive slip control.
Accordingly, this return pump, including a booster pump provided in association with the return pump, is activated as soon as a driven vehicle wheel, although it is not yet experiencing an excessive drive slip corresponding to the response threshold value of the drive slip control which demands "countering" by the control system, has reached a value of drive slip of this vehicle wheel which is higher, by a specified amount than the required value specified for the control system, so that it can be anticipated that this vehicle wheel will experience a still higher drive slip in the immediate future and will therefore be subjected to a control cycle. With the activation of the return pump, or slightly delayed relative to this activation, the brake pressure control valves of the driven vehicle wheels are also driven into their closed position and the drive slip control valve is driven into its position shutting off the main brake pipe from the brake device. By this means, a higher pressure is generated in the section of the main brake pipe of the brake circuit of the driven vehicle wheels branching towards the wheel brakes and this pressure can be connected to the wheel brake immediately when control on the vehicle wheel considered has to become effective, by switching the brake pressure control valve of this wheel into its brake pressure build-up position. The section of the main brake pipe branching off to the wheel brakes and a noise suppressor which is usually connected to it (this noise suppressor being normally provided only to buffer the pressure shocks from the return pump), is then used as a high pressure reservoir. Although the storage capacity of the latter is markedly less than the storage capacity of the pressure reservoir provided as the auxiliary pressure source in a drive slip control device of known type, it still permits the brake pressure build-up phase in the wheel brake of the particular vehicle wheel subjected to the control system. This build-up phase initiates the drive slip control cycle. Although the pressure level is only moderate, there is practically no delay and to this extent the build-up provides a desirably rapid response behavior to the drive slip control procedure during the further course of which the return pump can take over the build-up of the pressure to the necessary brake pressure level.
The control signals necessary for activating the return pump and for switching over the drive slip control valve and the brake pressure control valves are obtained from the electronic control unit of the drive slip control device by processing the output signals of the wheel speed sensors associated with the individual vehicle wheels according to known criteria. The wheel speed sensors generate electrical output signals characteristic of the dynamic behavior of the vehicle wheels in terms of level and/or frequency. These output signals cause the electronic control unit to generate an output signal for controlling the drive slip control valve, an output signal procuring the activation of the return pump and output signals driving the brake pressure control valves into their shut-off position as soon as the wheel peripheral acceleration of at least one of the driven vehicle wheels exceeds a response threshold value and/or is greater than the vehicle acceleration by more than a specified difference which is fixed or varies with vehicle speed.
By having the output signal of the electronic control unit cause the activation of the pump drive of the return pump of the brake circuit of the driven vehicle wheels as soon as the drive slip of least one of the driven vehicle wheels reaches or exceeds a specified threshold value, which is between 50% and 70% of the threshold value at which the drive slip control valve and the brake pressure control valves and of the wheel brakes of the driven vehicle wheels are moved into their shut-off position, and/or when the wheel peripheral acceleration of at least one of the driven vehicle wheels exceeds a response threshold value and/or is greater than the vehicle acceleration by a specified difference which is fixed or varies with the speed of the vehicle has the advantage that any influence of the pump running-up period is, so to speak, eliminated. The circulatory operation introducing a control cycle in this way generates a -dynamic-pressure which, although smaller, is still sufficient to bring the wheel brakes into contact so that when, subsequently, the brake device is shut off from the brake circuit, by driving the drive slip control valve, the wheel brakes respond with practically no delay, thus improving the sensitivity of the control system.
The arrangement of the drive slip control valve wherein the drive slip control valve is designed as a 2/2-way solenoid valve which is connected between the brake device pressure output associated with the brake circuit of the driven vehicle wheels and the section of the main brake pipe branching to the wheel brakes of the driven vehicle wheels and wherein the section of the main brake pipe branching to the wheel brakes of the driven vehicle wheels and a pressure limiting valve are connected in parallel with the drive slip control valve provides a particularly simple structure of the hydraulic unit in terms of switching and control.
Pressure loads on the brake device due to the control system are excluded by means of an additional outlet valve designed as a solenoid valve and provided as a further drive slip functional control valve by means of which the return pipe of the brake circuit of the driven vehicle wheels can be directly connected to the brake fluid reservoirs of the brake installation during the pressure reduction phases of the drive slip control, so that, in combination with the drive slip control device of the invention, a series production, simple design of brake device, e.g. a tandem main cylinder of conventional type, can be used.
The drive slip control device of the invention is achievable at substantially less cost than known equipment of this type and has, in addition, the advantage that the interference noises caused by the operation of individual elements of the control device only appear when the control device is activated in accordance with its purpose but not during normal driving, such as can occur during reservoir recharging phases in a conventional control device.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.